1. Field of the Invention
The invention relates to a method of forming conductive polymers. More particularly, the invention pertains to a method of forming head-to-tail coupled regioregular (rr) poly-(3-substituted) thiophenes. The invention also provides monomers suitable for the formation of regioregular poly-(3-substituted) thiophenes, regioregular substituted polythiophenes, as well as films and articles formed therefrom.
2. Description of the Related Art
As is well known, various materials can conduct electricity in different ways. For instance, metals conduct electricity by the movement of free electrons that are not tightly bound to any single atom. In semiconductors, like those that make up transistors and other electronic devices, electricity is produced by the drift of excess electrons that form a negative current, or alternatively the drift of missing electrons or positive “holes” in the opposite direction to form a positive current. Typically, these excess electrons or holes are donated by impurities or dopant atoms.
In the 1970s, it was discovered that polymers can be made to conduct electricity like metallic conductors and semiconductors. At the time, plastics were considered non-conductors, but it was discovered that adding impurities to a polymer material could increase its conductivity by more than a billion times. Today, the field of conducting polymers has been greatly expanded to a broad field of commercial applications.
Conducting polymers are finding increased use compared to other conductive materials because they are lightweight, highly processable and have good mechanical properties. Potential applications for conducting polymers include field-effect transistors, sensors, capacitor coatings, battery electrodes, light-emitting diodes, nonlinear optical materials, molecular wires and molecular switches. Among polymers that have shown conductive properties, polythiophenes are particularly desirable because of their excellent conductivity and processability. Of particular interest in the art are poly(3-alkylthiophenes) (P3ATs), including poly(3-hexylthiophene) (P3HT), which are attractive conductive polymer materials for many potential commercial applications because the alkyl side chains offer improved solubility in many common organic solvents, particularly ethers. Today, poly(3-alkylthiophenes) are widely used as hole-transporting materials in organic field-effect transistors.
To those skilled in the art of conductive polythiophene polymers, it is well known that the degree of conductivity exhibited by conductive polymers depends on their degree of order on a molecular level. This is due in part to a crystal lattice structure that allows an overlapping pathway for electrons. To illustrate, the conductivity of poly-(3-substituted thiophenes) is known to increase with the degree of regioregularity. Because of its asymmetrical structure, the polymerization of 3-substituted thiophene produces a mixture of polythiophene structures containing three possible regiochemical linkages between repeat units depending on the specific synthesis procedure. The three orientations available when two thiophene rings are joined are the 2,2′, 2,5′, and 5,5′ couplings. When application as a conducting polymer is desired, the 2,2′ (or head-to-head) coupling and the 5,5′ (or tail-to-tail) coupling, referred to as regiorandom couplings, are considered to be defects in the polymer structure because they cause a sterically driven twist of thiophene rings that disrupt conjugation, produce an amorphous structure, and prevent ideal solid state packing, thus diminishing electronic and photonic properties. The steric crowding of solubilizing groups in the 3 position leads to loss of planarity and less π overlap. In contrast, the 2,5′ (or head-to-tail (HT) coupled) regioregular polythiophenes can access a low energy planar conformation, leading to highly conjugated polymers that provide flat, stacking macromolecular structures that can self-assemble, providing efficient interchain and intrachain conductivity pathways. The electronic and photonic properties of the regioregular materials are maximized.
Various methods have been employed to synthesize 2,5′ regioregular polythiophenes. Two of the more commonly known methods are the “McCullough method”, described in U.S. Pat. No. 6,166,172 by Richard D. McCullough and Robert S. Loewe of Carnegie Mellon University, and the “Rieke method”, described in U.S. Pat. No. 5,358,546 by Reuben D. Rieke of the University of Nebraska. The McCullough method region-specifically generates 2-bromo-5-(bromomagnesio)-3-alkylthiophene from a monomer which is polymerized with catalytic amounts of 1,3-diphenylphosphinopropane nickel(II) chloride (Ni(dppp)Cl2) using Kumada cross-coupling methods. The Rieke method differs from the McCullough method primarily in the synthesis of an asymmetric organometallic intermediate. Rieke describes adding a 2,5-dibromo-3-alkylthiophene to a solution of highly reactive “Rieke zinc” to form a mixture of the isomers, 2-bromo-3-alkyl-5-(bromozincio) thiophene and 2-(bromozincio)-3-alkyl-5-bromothiophene. The addition of 1,2-bis(diphenylphosphino)ethane nickel(II) chloride (Ni(dppe)Cl2), a nickel cross-coupling catalyst, leads to the formation of regioregular HT-poly(3-alkylthiophenes). Each of these methods produce polythiophenes with a high percentage of HT couplings, in the range of 90% or higher. A detailed description of both the McCullough method and the Rieke method, as well as other methods, are illustrated in detail in U.S. Pat. No. 6,166,172.
Despite the efforts by those skilled in the art to improve HT coupling techniques, the synthetic procedures heretofore described have significant drawbacks. For example, the McCullough method requires highly purified starting materials, the most important of which is the monomer, 2-bromo-3-alkylthiophene. The need for purity adds to the cost of the synthesis. The Rieke method includes an easily purified 2,5-dibromo-3-alkylthiophene as a starting material, but requires the difficult preparation of Rieke zinc via alkali metal reduction of zinc halides in an inert environment. Accordingly, a new method for the preparation of regioregular, HT-poly-(3-alkylthiophenes) is needed that is efficient and economical. It has been unexpectedly found that poly(3-substituted) thiophenes formed with a thiophene monomer having two different halogen leaving groups will result in a thiophene polymer having superior conductive properties and at a higher yield and lower cost than other known processes. Additionally, poly(3-substituted)thiophenes of the invention have been found to have improved charge carrier mobility and current modulation (on/off ratio) properties compared to polythiophenes formed via prior art processes.